The present invention relates to an automatic control device for a helicopter, as well as to an automatic control system comprising such an automatic control device.
It is known that an automatic control system for a helicopter generally comprises:
a set of sensors for automatically determining the effective values of state parameters of the helicopter;
means of adjustment allowing an operator of the helicopter to choose preset values of state parameters;
a set of actuators which act on control axes of the helicopter, according to control commands; and
an automatic control device which automatically determines control commands for said actuators, as a function of the state parameters and of the preset values, received respectively from said sensors and from said means of adjustment.
The job of the automatic control system is to aid, or even to completely replace, the pilot in the flying of the helicopter. For this purpose, it is capable of slaving one or more state parameters of the helicopter, such as the altitude, the attitude or the speed for example, to one or more preset values chosen beforehand by the pilot. The automatic control system acts for this purpose on one of the control axes (roll axis, pitch axis, yaw axis and collective axis) so as to command the corresponding state parameter or parameters.
It will be noted that both the pitching axis and the collective axis make it possible to control the airspeed (which corresponds, according to the invention, to the speed of the helicopter with respect to the air) or parameters of the vertical plane. It will moreover be noted that the collective axis makes it possible to act on the pitch of the blades (as would the pilot by shifting the collective control stick) in such a way as to apply the same variations in angle of incidence to each of the various blades of the main forward motion and lift rotor of the helicopter. This results in a shift of the helicopter along a vertical axis (upward/downward).
Additionally, the two axes relating to the cyclic pitches (pitching axis and roll axis) produce a cyclic variation of the angle of incidence of the blades of the main rotor, these blades then having angles of incidence which vary as a function of their azimuth. This results in a forward tilting of the rotor (pitching) or sideways tilting (roll) or a combination of these two movements.
The automatic control system comprises various independent modes of slaving. Several of these modes may even be activated simultaneously. These modes are activated with the aim of carrying out a flight without intervention from the human pilot or pilots on the flight controls. Generally, an automatic control system is capable:
of acquiring and of maintaining a specified airspeed, that is to say a specified speed of the helicopter with respect to the air;
as well as of continuing an objective, in the vertical plane (acquisition and maintaining of a barometric altitude, of a vertical speed, of an approach slope or of a xe2x80x9cradioprobe heightxe2x80x9d for example);
whilst ensuring, in a horizontal plane, the following of a flight plan (alignment with a given heading, with guidance beacons, with let-down axes, etc.) and generally, steering along a trajectory.
Such a system for the automatic control of a helicopter which enables the pilot to free himself completely of the task of controlling the helicopter, for almost the entire flight, may exhibit safety problems, generally having two different types of origin:
a mechanical origin due to a malfunctioning of the set of sensors, computers and actuators forming part of the automatic control system; and
a human origin due to a lack of attention or to a lack of knowledge of the automatic control system and of its limitations on the part of a pilot or of a crew member of the helicopter.
For this purpose, it is known that the more the flight management is automated, the more the control task is eased, and the less inclined is the crew to monitor the proper operation of the control system and the consistency of the modes and presets which it itself has displayed. The automatic control system must therefore to a large extent ensure its own monitoring and introduce automatic protections or limitations to cover the risks of ill-use. Such monitoring and protection means are numerous in helicopter automatic control systems. The following will be cited by way of illustration:
a dual architecture of the control loop (sensor, computer, actuator) allowing multiple monitoring;
multiple alarms and signalings presented to the crew, allowing diagnosis of the state of the control system possibly calling for increased monitoring (indication of disparities between sensors, of loss of redundancy, of abnormal alteration or of excessive discrepancy in the event of deviation from the preset displayed, etc.), or even active participation (need for the pilot to regain manual control of the helicopter, etc.); and
flight domain protection elements, such as the limiting of commands heading for the collective axis so as not to exceed the maximum permitted power or automatic leveling-off at the end of an instrument approach using a set of beacons on the ground and of receivers on board the helicopter.
However, all these monitoring and protection means participate in essence in the safety of the functioning of the technical control loop, but do not directly reduce the risks related to the human factor, since the pilot still intervenes in the complete control loop (action on the controls, on the presets, on the selection of modes, etc.).
More especially, it is known that a standard automatic control device comprises:
a single objective law for the pitching axis, namely a forward motion objective law whose aim is to maintain the airspeed with respect to a preset value. To do this, said objective law for the pitching axis determines, automatically, a control command for operating the tilting of the disk of the main rotor of the helicopter. It will be noted that, within the framework of the present invention, an objective law is a means of calculation which determines a particular control command making it possible to bring or to bring back at least one state parameter of the helicopter to an objective (which represents a preset value of this state parameter or of another state parameter); and
two objective laws for the collective axis, namely a vertical objective law and a law of maintaining the recommended power. Each of these two laws determines, automatically, a control command for operating the collective pitch of the blades of the main rotor of the helicopter.
With such an automatic control device, during normal functioning of the helicopter, the acquisition and holding of the airspeed are achieved via the pitching axis (forward motion objective law) and that of the parameter of the vertical mode (for example the altitude) via the collective axis.
However, as soon as the available power becomes insufficient to maintain the airspeed (that is to say the speed of the helicopter with respect to the air) at the scheduled preset value, the vertical objective law of the collective axis hits a top limitation which is calculated by the law for maintaining the power (the object of which is to maintain the power at a recommended value). In this case, the automatic control device still ensures safety in terms of power (maintaining of the power required for the flight configuration), but more in terms of vertical objective (altitude for example), since the vertical objective is no longer commanded by this automatic control device. In such a situation, it is the pilot who must intervene to ensure safety in terms of vertical objective. Thus, when he notes the toggling of law to the collective axis and that the helicopter is still descending, the pilot disengages, manually, the mode for holding the airspeed on the pitching axis so as to release this axis for the achieving of the vertical objective.
This known solution therefore exhibits risks related to the necessary intervention of the pilot who must not only be aware of the situation, but must also react appropriately.
The object of the present invention is to remedy these drawbacks. It relates to a device for the automatic control of a helicopter exhibiting enhanced safety in fully automatic mode (with no intervention from a pilot), while making it possible to maintain the helicopter within an ever-safe flight envelope.
For this purpose, according to the invention, said automatic control device comprising:
at least one objective law in respect of the pitching axis, the aim of such an objective law being to bring at least one state parameter of the helicopter to an objective which represents a preset value of a state parameter, said objective law in respect of the pitching axis automatically determining for this purpose a control command for operating the tilting of the disk of said main rotor of the helicopter; and
at least two objective laws in respect of the collective axis, each of these laws automatically determining a control command for operating the collective pitch of the blades of said main rotor,
is noteworthy in that it comprises:
at least two objective laws in respect of the pitching axis, which function simultaneously;
first selection means for automatically selecting the objective law whose control command is used for the collective axis; and
second selection means for automatically selecting, as a function of the selection made by said first selection means, the objective law whose control command is used for the pitching axis.
Thus, by virtue of the simultaneous functioning of at least two objective laws relating to the pitching axis and of the use of the information regarding the switching of laws to the collective axis so as to select the appropriate law on the pitching axis, enhanced safety is obtained in automatic mode, making it possible to maintain the helicopter within a safe flight envelope, as will be seen in detail hereinbelow.
Moreover, this automatic switching is in accordance with what a pilot does naturally (and manually) in such a situation.
In a preferred embodiment, said automatic control device comprises at least one vertical objective law and one law for maintaining the recommended power for the collective axis, and at least one forward motion objective law and one vertical objective law for the pitching axis, said first selection means automatically select the vertical objective law during normal functioning of the helicopter and the law for maintaining the recommended power when the power becomes insufficient to maintain the airspeed of the helicopter at a corresponding preset value, and said second selection means automatically select:
the forward motion objective law, when the first selection means select the vertical objective law; and
the vertical objective law when the first selection means select the law for maintaining the recommended power.
Thus, by virtue of the invention, when the power of the helicopter is insufficient to maintain the airspeed at its preset value, the vertical objective (altitude, height, vertical speed, vertical acceleration, etc.) is ensured automatically by the pitching axis, whereas the collective axis automatically ensures the maintaining of the power at the recommended value. This is made possible by the simultaneous functioning of at least two objective laws (standard law with forward motion objective and vertical objective law) on the pitching axis and through the use of the information regarding the switch from the vertical objective law to the law for maintaining the recommended power on the collective axis, so as to select on the pitching axis the appropriate law, in this instance the vertical objective law.
This logic (or automatic switching) is in accordance with what a pilot does naturally in fast cruising flight.
Furthermore, advantageously, said automatic control device moreover comprises a law for maintaining the optimal upward speed for the pitching axis, and said second selection means automatically select said law for maintaining the optimal upward speed when the power available is insufficient to maintain the airspeed above an optimal upward speed.
This latter logic is likewise in accordance with what a pilot does naturally. It avoids a drift towards very low airspeeds, with considerable pitching attitudes, and ensures a minimal upward rate, regardless of the presets displayed by an operator (pilot, etc.) of the helicopter.
The aforesaid characteristics are made possible by virtue of the fact that the pitching axis which operates the forward tilting of the disk of the main rotor, makes it possible to control:
either a horizontal objective (airspeed for example) by virtue of the horizontal component of the aerodynamic resultant of the rotor disk (of said main forward motion and lift rotor of the helicopter);
or a vertical objective (vertical speed for example) by virtue of the vertical component of the aerodynamic resultant of said rotor disk.
Additionally, it will be noted that the insufficiency of power which is considered in the present invention, may have as object, in particular, a fault with one of the engines of the helicopter. Specifically, when one of the engines develops a fault, thereby obviously entailing an abrupt reduction in the available power, the automatic control device in accordance with the invention rapidly lowers the collective pitch, by applying its function of automatic limiting of the power to the recommended value, and automatically toggles the vertical objective on the pitching axis, thereby certainly reducing the airspeed, but avoiding any loss of altitude.
By virtue of the invention, no retaking of manual control by the pilot is therefore required during such a fault which is generally very critical since the pilot, who is nevertheless a priori distracted, is supposed to very quickly reduce the pitch, so as to prevent the rotor revolutions from dropping.
Thus, the present invention implements two characteristics specific to the limits of the flight domain, where safety may be threatened, namely:
a ranking of priorities. In complying with the recommended power, priority is given:
to the vertical objective, while the airspeed is greater than the optimal upward speed; and
to the airspeed preset, if it is below the optimal upward speed; and
combined consideration of the pitching axis and of the collective axis as regards the management of these priorities, and not independent use as in the case of a standard automatic control device, in which each control axis obeys just one objective.
Additionally, advantageously, said automatic control device moreover comprises latching means for automatically latching the collective axis to a law for maintaining the recommended power, when it is selected. This makes it possible to prevent oscillatory functioning of the helicopter.
In this case, said automatic control device moreover comprises means for automatically unlatching said latching means, as a function of at least one action of a pilot of the helicopter and/or of control commands which are determined for the pitching axis.
Additionally, it is known that in fast cruising flight, with a standard automatic control device, the airspeed is maintained via the pitching axis and the vertical objective is ensured via the collective axis. In this case, the fluctuations in the vertical plane (altitude, vertical speed, etc.) are therefore compensated for by the collective axis. Since these fluctuations are both very frequent and of fairly large amplitudes during high-speed flight, they make great demands on the collective axis, thereby entailing excess consumption of fuel. To avoid this, in practice, the pilot prefers to manage the power in fast cruising flight himself, but this then prevents him from being able to benefit from the aforesaid safety facilities offered by the automatic control device.
To remedy this drawback, according to the invention, the automatic control device comprises at least one vertical objective law and one stabilization law for the collective axis, at least one forward motion objective law and one vertical objective law for the pitching axis, and means for detecting stabilization of the case of flight (in particular level at substantially constant speed) of the helicopter. In addition, said first selection means automatically select in respect of the collective axis the vertical objective law, during normal functioning of the helicopter, and when said detection means detect stabilization of the case of flight, said stabilization law which determines a control command for maintaining the collective pitch of the blades of the main rotor at a mean value, and said second selection means automatically select:
the forward motion objective law, when the first selection means select the vertical objective law; and
the vertical objective law, when the first selection means select the stabilization law.
Furthermore, to counter a loss of speed effect which is contrary to the airspeed preset initially scheduled by the pilot, in particular during a maneuver (turning for example), said stabilization law comprises means for correcting the control command of the collective pitch, with a response time which is greater than a predetermined time, so as to maintain an airspeed objective. Since, according to the invention, this predetermined time is relatively long, the collective pitch is thus corrected gently.
Moreover, the automatic control device in accordance with the invention also comprises means for automatically returning in this case to the selection representative of normal functioning, as a function of at least one action of a pilot of the helicopter and/or of measured values.
The aforesaid logic exhibits two advantages:
on the one hand, it complies with what the pilot does naturally, it is therefore easy to interpret; and
on the other hand, the collective axis has only to compensate for the fluctuations in the airspeed (in particular while turning). Now, experience shows that their frequency and their amplitude are much lower than those of the fluctuations in the vertical plane. The aforesaid excess consumptions of fuel are therefore greatly reduced, thus prompting continuous use of the automatic control device throughout the nominal cruising domain, with all the inherent advantages in terms of safety in particular.
The present invention also relates to a system for the automatic control of a helicopter, of the type described above.
According to the invention, this automatic control system is noteworthy in that it comprises the aforesaid automatic control device.